If you're an Owner on a Stack you have the ability to assign yourself or another user Developer access.
Having an account type of Developer grants you access to the custom CSS, JS and HTML display editors.
To assign Developer access to a user:
Click on Users > Administrators and to the right of your own user account click on Edit in the Actions column.
On the next screen select the User Groups tab.
Click on the Developer User checkbox from the list of available roles, and then Save your changes.